$/ The input record separator, newline by default. This influences
Perl's idea of what a "line" is. Works like awk's RS variable,
including treating empty lines as a terminator if set to the
null string. (An empty line cannot contain any spaces or tabs.)
You may set it to a multi-character string to match a
multi-character terminator, or to "undef" to read through the
end of file. Setting it to "\n\n" means something slightly
different than setting to "", if the file contains consecutive
empty lines. Setting to "" will treat two or more consecutive
empty lines as a single empty line. Setting to "\n\n" will
blindly assume that the next input character belongs to the next
paragraph, even if it's a newline. (Mnemonic: / delimits line
boundaries when quoting poetry.)

All systems use the virtual "\n" to represent a line terminator, called
a "newline". There is no such thing as an unvarying, physical newline
character. It is only an illusion that the operating system, device
drivers, C libraries, and Perl all conspire to preserve. Not all sys-
tems read "\r" as ASCII CR and "\n" as ASCII LF. For example, on a
Mac, these are reversed, and on systems without line terminator, print-
ing "\n" may emit no actual data. In general, use "\n" when you mean a
"newline" for your system, but use the literal ASCII when you need an
exact character. For example, most networking protocols expect and
prefer a CR+LF ("\015\012" or "\cM\cJ") for line terminators, and
although they often accept just "\012", they seldom tolerate just
"\015". If you get in the habit of using "\n" for networking, you may
be burned some day.

So in other words, use "\n" for your local system's view of a newline, or use actual characters if you know what you want, regardless of what system you're running on.

Just to complicate things further, please note that this is out of date and somewhat inaccurate. When perldoc says "on a Mac," it means "on a Mac running the Macintosh operating system up through version 9." Mac OS X (version 10) and newer use Unix-style newlines.